The present invention relates to a method of detecting defects, if any, of at least one rotor of a rotary wing aircraft, in particular a helicopter, and for adjusting the rotor. The invention also relates to apparatus for determining defects and adjustment values for adjustment parameters of such a rotor.
In the context of the present invention:                the term “detecting defects of the rotor” is used to mean detecting defective parts of said rotor (e.g. in non-exhaustive manner, a ball bearing, a lag damper, an undercut leading edge of a blade, a pitch-link or any other mechanical part presenting a defect, that might generate an increased level of rotor vibration), which elimination of said defects corresponding to replacing the defective parts; and        the term “adjusting the rotor” is used to mean adjusting particular elements of the rotor (e.g. compensating tabs or weights mounted on the rotor blades) for the purpose of reducing and minimizing the vibration of at least one portion (e.g. the pilot's seat) of a rotary wing aircraft.        
Such vibration constitutes a major problem that needs to be countered, since such vibration leads to:                alternating stresses throughout the aircraft giving rise to fatigue phenomena, and thus having a direct influence on safety;        vibrations in the fuselage which can reduce the precision and the effectiveness of equipment, in particular weapons, mounted on said fuselage; and        vibrations in the cabin, which is naturally very troublesome for the comfort of the pilots and the passengers.        
Document EP-0377 666 (U.S. Pat. No. 4,937,758) discloses a system and a method for minimizing vibration and stress within an apparatus such as a helicopter having a rotor with rotary blades disposed thereabout. Said method includes the steps consisting in (and the system includes means for):
a) calculating the effects of unit mechanical adjustments on the force and the moment exerted by the rotor on the support structure. Each specific helicopter possesses its own computer file, which remains with the helicopter throughout its lifetime. Thus, each helicopter with a serial number has its own particular database (i.e. which applies to that helicopter only) containing a vibration profile of the helicopter over time, accompanied by descriptive information relating to flight test conditions and to maintenance actions;
b) determining at least three force components and at least three moment components generated by the rotor;
c) detecting the angular position of a shaft of said rotor;
d) processing the signals representing the force and moment components and the signals representing the angular position of said shaft in order to produce the Fourier coefficients of said forces and moments;
e) determining the optimum mechanical adjustments for the rotor in order to minimize the vibrations in the support structure of the rotor, said optimum adjustments being deduced from said Fourier coefficients of the movements produced and from said calculated effects of the unit mechanical adjustments of the blades; and
f) prescribing blade adjustments in agreement with said optimum mechanical adjustments.
The method of minimizing vibration, as described in that document EP-0 377 666 presents the following characteristics in particular:
on each occasion it applies to a single helicopter only, for which it is necessary to begin by calculating the effects of mechanical adjustments on the force and the moment exerted by the rotor;                it requires forces and moments to be obtained that are due firstly to a non-adjusted rotor and secondly to each of the various adjustments. These forces and moments can be deduced from measurements performed by measurement means such as accelerometers or strain gauges fixed on the structure, with the deformations thereof being measured;        it implements an approximation by assuming that the fuselage of the helicopter is a rigid body presenting six degrees of freedom (whereas, in fact, the helicopter is a deformable body which is subjected to external excitations coming in particular from the rotors and from various aerodynamic forces), and it seeks to correct the movements of said rigid body in three dimensions;        in order to be implemented, it requires knowledge of the exact locations in the helicopter cabin of said measurement means (accelerometers, strain gauges, . . . ) relative to the center of gravity of the helicopter; and        in order to calculate the mechanical adjustments, it uses an ordinary linear system with influence coefficients.        
That prior art method (or system) presents several major drawbacks:
A/firstly, by assuming that the helicopter is a rigid body and thus by trying to minimize the vibrations of its structure without taking account of any possible deformations thereof, said prior art method is not capable of achieving optimum adjustment of the rotor;
B/secondly, as mentioned above, it is necessary to know the actual exact location of the measurement means relative to the center of gravity of the helicopter.
Consequently, there must be no errors made while putting said measurement means in the locations that are provided for this purpose.
In addition, as the position of the center of gravity of the helicopter varies as a function of its loading, and since the locations of measurement means that are installed once only, any subsequent variation of the weight or the weight distribution within the helicopter will lead specifically to errors in implementation of the method, thereby reducing the accuracy of the adjustment achieved; and                C/thirdly and above all, that prior art document recommends adjusting a helicopter rotor for the purpose of reducing vibration without taking any account of the defects, if any, of the rotor, i.e. without verifying whether parts such as the ball bearings or the dampers of said rotor are defective and are responsible at least in part for the vibration that is to be reduced.        
It is known that with defects of the above type, the intensity of the vibration due to the defects varies, generally as a function of the speed of rotation of the rotor. Consequently, by using the method disclosed in document EP-0 377 666 to adjust a rotor presenting at least one such defect, while taking no account of the defect, the vibration is indeed usually reduced for the speeds of rotation that were taken into account for the adjustment, but vibration is not minimized for all speeds of rotation. On the contrary, the adjustment can even have the opposite effect, i.e. under certain conditions, it can lead to vibration being increased at other speeds of rotation.
Furthermore, it can happen that the vibration due to a defect of the rotor is so great that it becomes impossible to identify the influence matrix that enables the above-mentioned method to be implemented.
Consequently, when defects exist on the rotor, the solution recommended by document EP-0 377 666 is generally not applicable since:                it is generally not possible to reduce the levels of vibration throughout the range of flying conditions in order to guarantee satisfactory comfort; and/or        it is often very difficult and sometimes impossible to identify the influence matrix needed for implementing said method.        